Such an X-ray examination apparatus and filter are disclosed in DE 689 868. In this Patent Specification a filter is described for use in a medical X-ray examination apparatus, the filter being formed by a liquid space between two flexible walls. The filter is arranged between an X-ray source and an X-ray detector. An X-ray absorbing liquid can be inserted between the walls. The walls have such a curvature, that X-ray radiation received from an X-ray focus is spatially uniformly attenuated by the filter. The path length of X-rays passing through the filter at its edges, is equal to the path length of X-rays passing through the center portion of the filter. The filter has for its object to remove the low-energetic X-ray radiation, which contributes only little to the image being formed, from the beam. This reduces the dose of radiation to which a patient is exposed during a medical examination.
In X-ray examination apparatuses in which an X-ray image is displayed as a televison picture on a monitor with the aid of an X-ray image intensifier tube and a television pick-up device coupled thereto, vignetting occurs due to the geometry of an input screen of the X-ray image intensifier tube and the presence, if any, of optical means between the television pick-up device and the X-ray image intensifier tube. As a result thereof, the edges of the X-ray image become darker than center portions of the X-ray image, even in the case of an uniform irradiation of the X-ray image intensifier tube. The invention has inter alia for its object to provide an X-ray absorbing filter, which reduces the spatial brightness variation in an X-ray image, produced by vignetting.